The recent discovery of high temperature superconductive materials has triggered tremendous research effort in chemistry, physics and processing of these ceramic materials. While the search for new high T.sub.c materials still continues and possible superconductivity at 200.degree. K. in these materials is projected the research effort to this date has already produced a number of materials which show significant promise for applications. These include the 95.degree. K. Y-Ba-Cu-O material, the 105.degree. K. Bi-Ca-Sr-CU-O material, the 125.degree. K. Tl-Ca-Ba-Cu-O material and their copper-oxide based variations.
The vast majority of the bulk high T.sub.c materials are presently synthesized by solid state reaction This synthesis method, however, results in significant local inhomogeneity in composition and phase, thereby requiring subsequent separation and purification. Wet chemical synthesis from organometallic precursors is known to reduce the local inhomogeneity in the bulk materials. However, this synthesis method has accomplished only limited success to this date mainly due to the limited availability of appropriate precursors. Moreover, there is growing evidence that the extremely small colloidal particles produced by this method, typically in the size range of 500-1000 A, are too small to induce superconductivity as individual grains.
The critical current density J.sub.c is one of the most important parameters of a superconducting material from an applications standpoint. Critical current densities in the order of 10.sup.5 -10.sup.6 A/cm.sup.2 are desired in bulk superconductors for the vast majority of applications including those for microelectronics. To date, the maximum value obtained with bulk samples is still two orders of magnitude lower than that needed for large scale applications.
Initially, superconductivity in the ceramic high T.sub.c compounds was thought to be an isotropic bulk phenomenon. However, there is increasing evidence that anisotropic layers have a significant effect. For example, Garcia and co-workers, in Phys. Rev. Lett., 60, 744 (1988), have recently reported that isolated single grains of Y-Ba-Cu-O material did not levitate in a magnetic field gradient However, a bulk specimen made from these grains did levitate in the same magnetic field These researchers attributed these observations to the localization of superconductivity in the twin boundary region. These twin boundaries in isolated grains will align when placed in a magnetic field gradient. Such alignment results in a minimum Meissner effect force. However, when particles are agglomerated, there is no preferred direction of alignment and the Meissner effect force is significant.
The Meissner effect, first discovered in 1933, generally refers to almost complete exclusion of magnetic flux from a bulk specimen in the superconductive state. This effect has been often demonstrated graphically in terms of certain repulsive forces resulting from the magnetic flux exclusion Two such graphical demonstrations are shown in the book Superconductivity, by A. W. B. Taylor, published by Wykehom Publications (London) Ltd. 1970, in which a permanent magnet is shown to levitate above a superconductive material and in which a superconductive sphere is shown to float over a solenoid coil. In each case, the repulsive force created by the magnetic flux exclusion are shown to be balanced by the weight of the floating object (which is the magnet in the first case and the superconductive sphere in the second case).
Recent developments in the area of superconductivity have lead to the synthesis of new ceramic materials that are superconductive at temperatures T.sub.c as high as 125.degree. K. However, synthesis of superconductive ceramic materials often does not result in uniform superconducting particles. Some researchers in the field have reported from 24% to 50% non-superconductive particles in each batch synthesized. Further, while some of the particles are non-conductive, others of the particles are partially superconductive, and partially superconductive by different amounts The scientific community is presently unable to provide a conclusive theory or mechanism for the superconductivity of these new compounds. Further, future development of superconductor technology will require effective methods and means for separating and classifying superconductive particles.
U.S. Pat. No. 4,526,681 is representative of a large number of patents which disclose the separation of magnetically susceptible particles in a colloidal suspension by the application of a magnetic field to form a magnetic susceptibility gradient.
U.S. Pat. No. 4,235,710 describes a magnetic separator having rounded poles to maximize the magnetic energy gradient of the device. It is illustrative of commercially available Franz magnetic separators.
The article A Magnetic Control Valve for Flowing Solids: Exploratory Studies by Yang, et al., appearing in Ind. Eng. Chem. Process Des. Dev., Vol. 21, No. 4 in 1982 discloses a flow control valve for controlling the flow of magnetically susceptible solids in a pipe by the application of a magnetic field to a conductive grate and iron screen disposed across the pipe.